Capacity motion responsive device



1386- 1952 J. w. GRAY CAPACITY MOTION RESPONSIVE DEVICE Filed June 10,1948 2 SHEETSSHEET l FIG-L2.

INVENTOR JOHN W. GRAY ATTO Dec. 30, 1952 J. w. GRAY 2,623,996

CAPACITY MOTION RESPONSIVE DEVICE Filed June 10, 1948 2 SHEETSSHEET 2 8SI 97 84 i 038 Has :15

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INVENTOR I JOHN W. GRAY BYga/A%$v Patented Dec. 30, 1952 CAPACITY MOTIONRESPONSIVE DEVICE John W. Gray, White Plains, N. Y., assignor to GeneralPrecision Laboratory Incorporated, a corporation of New York ApplicationJune 10, 1948, Serial No. 32,091

6 Claims.

This invention relates to a linear capacity motion responsive device.More specifically the invention relates to a capacitive or electrostaticmechanism associated with suitable electronic circuits whereby a givenmechanical motion is translated into an electrical quantity which isdirectly and linearly proportional to the degree of mechanical motion.

Many different arrangements have heretofore been proposed to perform thegeneral function of translating small mechanical displacements intoelectrical quantities which may then be used to directly operate anindicating instrument or through the medium of other apparatus act as ameans for remotely controlling various types of instrumentation as intelemetering systems such as servo controls or the like.

In general the devices heretofore proposed have depended onelectromagnetic phenomena to perform the desired purposes. Such devices,however, require specially wound coils so that true linearity ofresponse is obtained; they are affected by stray magnetic fields whichare particularly difficult to guard against in many industrialinstallations and they are affected by temperature variations andsaturation effects.

The instant invention depends on electrostatic phenomena and is notaffected by heat so that it may be used where high temperatures existwithout destroying its accuracy of operation. Likewise, sinceelectrostatic fields are easily shielded against, the device may easilybe protected from stray fields that may exist in any particularinstallation. Additionally the moving portions of the apparatus may bemade very light so that little force is required for the operationthereof and the apparatus may have a very low inertia.

To accomplish these functions and purposes, the instant inventioncontemplates the provision of a movable plate or probe which is variablypositioned between two fixed plates dependent upon the degree ofmechanical movement or deflection imparted to the movable plate. Thefixed plates are energized by a voltage so that in moving across thespace between the fixed plates, the movable plate assumes the potentialthat would exist at any location if this plate or probe were not there.In other words, as the movable plate is caused to traverse the path fromone fixed plate to another, the potential of the movable plate variesfrom the potential of one to that of the other in a linear fashion.

In order that this linearity may be maintained and that there be nodistortion of output it is essential that no appreciable current betaken from the probe plate and that there be no capacitive or resistanceloading in the circuit connected to the probe plate. An essentialfeature of the present invention, therefore, resides in the provision ofcircuit arrangements whereby these requirements are satisfied.

In the instant invention amplifier systems have been devised whichutilize negative feedback circuit or circuits in such a fashion thatpractically zero current is taken from the probe plate and the amplifierdelivers whatever power is required in any particular application.

The very fact of connecting an electrical conductor between the movableprobe plate and the input of an amplifier would ordinarily result insuch a capacity and resistive loading thereof as to introduce so muchdistortion as to render such a device of little utility for its intendedpurpose. The conductor connecting the probe plate and the input of theamplifier has a certain capacity as respects ground and the tubeelements themselves have an internal capacity which would constitute anadverse load upon the probe plate. In the instant invention the negativefeedback circuits used nullify and eliminate these adverse influences sothat the potential assumed by the probe plate at any position in itsmovable range may be converted to a useful power output without anydeleterious loading effects which would operate to produce an outputdeparting from true linearity The exact nature of the invention will bemore fully appreciated from a consideration of the following detaileddescription when taken together with the attached drawings, in which:

Figure l is a schematic diagram of one form of the invention.

Figure 2 is a schematic diagram of a modified form of the invention, and

Figure 3 is a schematic diagram of a further modified form of theinvention.

Figure 4 is a schematic diagram of a portion of the apparatus in whichthe capacity motion responsive device is energized by a source of directcurrent.

Referring now to the modification disclosed in Fig. 1, a thin movableplate or electrostatic probe I3 is mounted between two fixed plates l land [2. The movable plate l3 may if desired be made somewhat smallerthan the fixed plates l l and I2 so that the fringing of theelectrostatic field at the edges of these plates does not affect thelinearity of potential of the movable plate as it moves from a positionadjacent one plate to a position adjacent the other. On the other handthe fringing effect may be counteracted by suitable shaping of theoutside plates. In either event it is important that the potential ofthe movable plate in its travel across the space between the fixedplates be a true linear function of the position thereof and this may beaccomplished by either of the methods just mentioned.

Linear movement may be imparted to the movable plate i3 in any desiredmanner and the mechanism for impartin such movement is here illustrateddiagrammatically by a rod l 4 movable in a direction longitudinal ofits. length .in guides E6. The specific mechanismwhich imparts therequired movement forms no part of the instant invention and it may takeany one of a multitude of forms depending in large measure on which ofany one of a great number of applications to which the instant inventionis put. For example, the rod [4 may be connected to a pressure diaphragmor other pressure or differential pressure measuring apparatus toprovide an output which is a linear function of pressure or difierence.in pressure and in such instances the travel may be. madev very smallif connected to delicate instruments such as aneroid barometers, forexample, .01 inch.

On the other hand, the mechanism might well be used .for such a widelydifierent purpose as a phonograph pickup, the exciting voltage being ofintermediate frequency so that the output may be delivered to theintermediate stages of a radio receiver thereby reducing troublesome lowfrequency pickup that is sometimes bothersome in radio phonographcombinations. In such an instance the rod It would be actuated by thestylus of the phonograph pickup the movement of the plate It beinimparted by the movement of the stylus in the record groove.

Whatever the use, which as the two examples just given indicate coversan extremely wide range, the plate i3 is made to move through the spacebetween the fixed plates l I and [2 by such mechanism and in such amanner as the circumstances of use dictate."

The fixed'plates II and 52 are energized from a source of alternatingcurrent voltage l7 through a transformer 13, the opposite ends of thesecondary is of which are respectively connected to plates I land I 2.An intermediate point of the secondary It is grounded through the mediumof potentiometer 2 l, movable contact 22 and conductor 23. The exactintermediate point of the secondary which is placed at ground potentialwill depend on the position of contact 22 on the potentiometer 2i andhence the point of zero potential between the plates Hand l2 may beadjusted over a desired range for purposes that Will presently appear. 7h Assuming for the moment that the contact 22 is at the midpoint of itsrange, fixed plates ii and I2 will have impressed thereon alternatingcurrent potentials of equal amplitudes and opposite phase and these maybe made as high as is desired so that little or no voltage amplificationis required of the remainder of the system. If under such circumstancesthe movable plate 53 is exactly midway between the plates ii and i 2 nopotential will be impressed thereon since this point in the spacebetween the two fixed plates l l and i2 is a point of zero electrostaticpotential. Any movement of the plate is to one side or the other,however, will cause this plate or probe to assume an alternating currentpotential whose amplitude is directly proportional to the degree ofmovement from the mid-point. That is to say, the potential of the plateIt will vary in amplitude in a continuous manner from that of the plateI I when it is in contact therewith, through zero, to a potential of theopposite phase when the plate l3 contacts plate [2 in the movement ofplate l3 through its entire range. I

In commercial manufacture it may be difficult and economicallyunfeasible to provide a mechanism in which at the zero or at restposition the plate I3 is located exactly midway between the plates H andi2. To allow for manufacturing tolerances and to offset anydiscrepancies resulting therefrom, the system utilizes the poteniometer2| and movable contact 22. By adjusting the position of contact 22 thepoint of zero potential may be varied in its position between the fixedplates II and I2 so as to be made to correspond with any desired. orassumed position of the plate [3, thereby constituting a zeroadjustment. If such a refinement is not desired the secondary 19 may begrounded at its midpoint and in that case the point midway between theplates H and 12 will always be the point of zero potential.

The space between the plates may be an air space or it may be filledwith liquid of as high a dielectric constant as is desirable in anyparticular instance.

The probe plate It is connected to the control grid 2d of a pentode 26which is provided with a high unbypassed resistance 21 in its cathodelead providing a large amount or" negative feedback for the cathode 28and causing the tube to be operated as a cathode follower. Theleadconmeeting the probe plate is andthe control grid is provided with ashield 29 which may also shield thegrid resistor 3 I.

An inherent capacity exists between the grid lead and its shield 29 andalso between the control grid 2 cathode 28, suppressor grid 32 andscreen grid 33 which if not eliminated or at least minimized. to a largeextent constitutes a capacity load on the probe plate I 3 whichseriously affects the desired linearity of response.

In the present instance the shield 291s connected to the cathode side ofthe re'sistor'Z'I through a conductor 3d and the screen grid 33 isconnected to the same point for alternating currents by a condenser '35.Additionally, the su pressor grid 32 is directly connected to the oathode 28. Theplate is directly connected tothe positive side of apotential supply source, here indicated as a battery 38 for simplicityand clearness of illustration, and the negative side of this source isconnected to ground to which is also connected the end of resistor 2'!remote from the cathode 28. The potential drop occasioned by the outputof the tube 26 therefore occurs across this resistor and the output maybe derived from terminals ll, 32 to be utilized in any desired manner.At the same time this potential drop serves to increase the potential ofthe cathode 23 as respects ground so that any increase in con trol gridpotential is accompanied by an almost equal increase in cathodepotential. In other words, the potential of the cathode follows that ofthe control grid. This is a well understood phenomenon of cathodefollower or negative feedback circuits of this type, the voltage gainalways being very slightly less than unity.

Inasmuch as the potential of the cathode always closely approaches thatof the control grid W e it may be, asoccasioned by the relative positionof the movable'plate' 'lfwitli respect to the fixed plates H and I2, theshield 29, suppressor grid 32 and screen grid 33 which are connected tothe cathode likewise follow the changes in control grid potential. Theactual capacitance between the control grid and these electrodes,therefore, is not effective as a load on the probe plate. Likewise theloading eifects of the grid resistor 3| and shield 29 are nearlyeliminated. That is, since these elements are always made nearly thepotential of the grid 24 and probe plate 13 there is very littlepotential difference existing between the grid lead and its surroundingshield and the effect of the capacity existing between these elements isreduced in like proportion to the reduction in potential difference.

The tube 26 of course, has a voltage gain of less than one and thereforethe potential existing at the output terminals 4| and 42 is not as greatas the potential of the probe plate H but since as much alternatingvoltage as is desired may be impressed across the fixed plates II and I2the lack of voltage amplification is not a problem.

In such cases where the lead from the plate [3 to the control grid 24 isquite long so that the capacity between the grid lead and its shield 29is great the slight residuum of potential difference existing betweenthe grid lead and its shield, as a result of the fact that the voltagegain of the tube 26 is slightly less than unity, may cause enoughresidual loading to produce noticeable nonlinearity.

In such cases, i. e., where the grid lead must be long, it is preferredto use the arrangement as illustrated in Fig. 2 wherein a two stageamplifier is utilized in such a fashion that the impedance as seen bythe probe plate is made exactly infinite and no distortion is producedin the capacity responsive device.

In the circuit of Fig. 2 the capacity responsive mechanism is the sameas that discussed in connection with the system of Fig. 1 comprising apair of fixed plates 46 and 41 with a movable plate 43 locatedtherebetween, the movable plate being actuated in any desired manner asdiscussed in connection with the modification of Fig. 1.

The fixed plates 46 and 41 are energized by an alternating currentsource 49 through the medium of a transformer 5| and a zero adjustmentmay be provided by connecting the potentiometer 52 having a movablecontact 53 in parallel with the transformer secondary as heretoforedescribed.

The probe plate 48 is connected to the control grid 54 of a two stageamplifier consisting of pentode 51 and triode 58 through a lead 56. Anelectrostatic shield 59 is provided for the lead 56 and this shield mayalso encompass the grid resistor 6|.

The output of the initial amplifier stage 51 is coupled through aresistance capacity coupling circuit to the input of the second andfinal stage 58 and the voltage output is derived from terminals 62 and63 connected to the output circuit of the final stage 58 which outputcircuit includes a resistor network comprising resistors 64, 65 and 66connected in a series parallel combination between the output terminals.

In such an arrangement amplified alternating current voltage drops willbe developed across the output resistor network which vary in the samesense as the potential applied to the control grid 54 of the firstamplifier stage 51 and suitable negative feedback voltages are derivedfrom this network to accomplish the function of eliminating loadingeffects on the probe plate 48 so that the output as derived fromterminals 62 and 63 is at all times a true linear function of therelative position of the probe plate 48 as respects the fixed plates 46and 41.

The main negative feedback loop for stabilizing the amplifier andpreventing distortion is formed by connecting the cathode 68 of the tube51 to the terminal 61 on the resistor network composed of resistors 64,65 and 66, conductor 69 providing this connection. No voltage gain inthe system is necessary for the reason that the alternating currentpotential applied to the fixed plates 46 and 4! may be as high as isdesired, and the only function of the amplifier is as a power source andas a means for preventing as far as possible any current drain on themovable probe plate 48 so that the output is a true representation ofthe electrostatic potential of this member. Because no voltage gain isnecessary the amount of negative feedback applied to the cathode 68 maybe large and the cathode 63 may be made to approach very closely thealternating current potential of the grid 54. In other words, the signalpotential existing between grid and cathode of tube 51 may be very lowfor a given output. Under these conditions the input admittance of thetube 51 is greatly reduced preventing the imposition of a load on theprobe plate 46 as a result of such input admittance.

In addition to the -main negative feedback circuit constituted by theconductor 69, a feedback circuit to the grid resistor 6| is establishedby the conductor H connected to the movable tap 72 on resistance 64 anda feedback circuit to the grid shield 59 is established by the conductor13 connected to the movable tap 14 on resistor 65. These auxiliaryfeedback circuits are adjustable so that the grid lead 56 will drawexactly zero current either in phase or in quadrature with the impressedvoltage, slightly greater voltages being fed back to these elements thanto the cathode 6B.

The feedback to the grid resistor 6| is adjusted by varying the positionof the contact 12 until the feedback potential applied to the lower endof resistor 6! is just equal to the grid voltage. Under this conditionno current flows in this resistor to act as an in phase drain on theprobe plate 48.

The feedback to the shield 59 is made slightly higher than the feedbackto the grid by suitably adjusting the contact 54 on the resistor 65 inorder that the shield 59 shall be slightly increased in alternatingcurrent potential over that of the grid lead 56, thereby providingenough effective negative capacitance betwen grid lead and shield tocancel any residual unshielded capacitance to ground from the grid leador grid, and by this means any in quadrature drain of current from theprobe plate 48 is prevented.

By the use of these feedback circuits'the impedance lcoking from theprobe plate 46 becomes infinite so that the output derived therefrom isa true linear representation of the probe position as it would be ifsuspended between the plates 46 and 4'! without any electricalconnections being made thereto. That is to say, the deleterious effectswhich would ordinarily occur because of the necessary attachment ofelectrical circuits to the probe to obtain an indication of itselectrostatic potential are eliminated.

In the modification as disclosed in Fig. 3 a somewhat differentarrangement of negative and a direct connection between said shield anda third intermediate terminal on said resistor network the potentialamplitude of which is greater than the potential amplitude of saidsecond intermediate terminal.

6. A capacity motion responsive device according to claim 5 in whichsaid first discharge tube additionally includes a suppressor grid and ascreen grid, said suppressor grid being directly connected to saidcathode and said screen grid being capacitatively connected to saidcathode.

JOHN W. GRAY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Name Date Lenehan May 17, 1938 Number OTHERREFERENCES Electronics, vol. 10, January 1937, Feedback AmplifierDesign, by Terman, pages 12-15 and 50.

